This invention relates generally to apparatus and methods for measuring flow rates of fluids. In particular, the invention relates to an improved vortex flowmeter for measuring flow velocity of fluids.
Vortex flowmeters typically have a tubular passage, such as a pipe, for guiding a flowing fluid, herein referred to as process fluid, and have an obstruction element, also termed a vortex shedder, interposed in the path of the flowing fluid. The obstruction element includes a blunt surface facing the fluid flow for creating a series of spaced vortices downstream in the flowing fluid and includes a xe2x80x9ctailxe2x80x9d section for stabilizing the vortices. Under certain conditions, the vortex shedder creates two nearly-parallel rows of vortices on opposite sides of the shedder. These vortices are known as a Van Karman vortex street. The vortices in one row are staggered with respect to the vortices in the other row. It is understood that the frequency of these generated vortices is linearly proportional to the average flow velocity of the fluid. Thus, a measurement of the frequency of the vortices provides a measure of the average flow velocity of the fluid. A vortex-responsive sensor detects the pressure fluctuations associated with the passage of the vortices and drives an electronic unit that determines the frequency of the vortices, to determine the flow velocity of the fluid.
Vortex shedders of some conventional flowmeters are integrally cast as one unit with the pipe within which the shedder resides. As the size of the vortex shedder increases, or at very small sizes, such integral casting process is not practical. Thus, in many conventional flowmeters, the pipe and the vortex shedder are manufactured separately, and subsequently assembled together. The manufacture of such a conventional vortex flow meter typically consists of providing a pipe that is machined to accommodate a separate mounting plate, and to include a small locating hole diametrically opposed to the mounting pad for precise positioning of a shedder. In many conventional vortex flowmeters, the obstruction element spans the entire inner diameter of the pipe that contains the process fluid. The manufacturing process typically produces the shedder through precision casting. The cast shedder has curved ends having radii that are machined to conform with the inner curvature of the pipe. The machined shedder typically includes a locating hole on one end and a recess adjacent the other end for accommodating a sensor.
The assembly of such a shedder within the pipe includes the steps of aligning the radial extension of the shedder within the pipe by placing the locating hole of the shedder in register with the locating hole of the pipe, and inserting in the registered holes a set screw or like pin members to maintain the alignment of the shedder. The shedder is then welded to the inner surface of the pipe along both of its curved ends. The mounting plate is located precisely over the shedder and welded in place.
Such conventional flowmeter construction suffers from a number of drawbacks. In particular, the locating steps for alignment purposes can be time-consuming and costly. Further, any flaw in the alignment steps can adversely affect the flowmeter operation. For example, slight changes in the alignment of the shedder within the pipe can adversely impact the linearity of the flow meter. Another disadvantage of the conventional construction is that the process of producing curved ends for the shedder, especially for large shedders that fit within pipes having inner diameters of approximately 6 inches (15 cm) or larger, is cumbersome and timeconsuming.
Shedders are typically dimensioned such that they are not in intimate contact with the inside of the pipe before welding. A weld material, applied around the perimeter of each end of the shedder to within approximately a fraction of an inch of the blunt surface and the tail section of the shedder, holds the shedder in place. The resulting gaps between the blunt surface and the pipe, and between the tail portion and the pipe are spanned by small tack welds. Such tack welds are susceptible to failures, for example due to mechanical stress, that loosen the shedder with a concomitant loss of accuracy.
Another conventional vortex flowmeter structure accommodates thermal expansion of the shedder within the pipe by providing gaps between the wall of the fluid-guiding pipe and the shedder. Nevertheless, thermal expansion of the shedder in such vortex flowmeters can lead to weld cracking, and concomitant loosening of the shedder.
Another disadvantage of conventional designs is that separate shedders are required for pipes having different sizes. For example, the pressure of the flowing fluid can vary from one application to another. As the application pressure increases, the thickness of the wall of the pipe for guiding the fluid needs to increase to accommodate the increased pressure. Industry standards typically hold the outer diameter (xe2x80x9cODxe2x80x9d) of the pipe constant, and adjust the inner diameter (xe2x80x9cIDxe2x80x9d) in order to change the wall thickness. The change in the ID necessitates a different shedder, having a different length, to fit within the pipe.
It is thus an object of the invention to provide a vortex flowmeter that is less costly to manufacture than conventional flowmeters of comparable performance.
It is another object of the invention to provide a vortex shedder that is adaptable to a number of flowtube sizes.
It is another object of the invention to provide a flow meter that is less susceptible to loosening of the shedder as a result of mechanical and/or thermal stress.
It is a further object of the invention to provide a vortex flowmeter that can be configured for use with different types of fluids.
Other general and specific objects of this invention will in part be obvious and in part be evident from the drawings and description which follow.
The invention attains the foregoing and other objects by providing an improved vortex-sensing apparatus for measuring flow velocity of a fluid. The flowmeter of the invention includes a conduit having a wall that forms a passage for guiding the fluid. The wall includes two opposed openings that allow passage of a vortex-creating obstruction element therethrough. The opposed openings can be positioned along any chord of the pipe cross section. A preferred embodiment places the opposed openings along a central chord, i.e., diameter of the pipe. For illustration purposes, the embodiments of the invention are described herein by reference to diametrically opposed openings. The obstruction element includes an elongate portion, and two mounting pads, each of which is integrally connected to a different end of the elongate portion. The opposed openings allow insertion of the obstruction element into the passage such that each mounting pad partially protrudes externally of the pipe, through the openings. The flow of the process fluid past the obstruction element produces two streams of spaced vortices, with the vortices in one stream staggered or spatially offset with respect to those in the other stream. A sensor element detects the vortices, thereby measuring the flow velocity of the fluid.
One preferred practice of the invention bevels both pipe openings, or otherwise prepares the areas to be welded for accepting the weld deposit, externally to allow a full penetration weld of the obstruction element to the outside surface of the wall of the conduit. The welding of the shedder externally to the conduit maintains the rigid disposition of the shedder within the passage. In addition, the full penetration weld of the shedder to the pipe creates a pressure-tight attachment of the shedder to the pipe, and structurally reinforces the pipe against internal pressure.
As is conventional, in a vortex flowmeter for practice of the invention, the elongate portion of the shedder includes a blunt surface, and a tail portion. The disposition of the shedder within the passage is preferably such that the blunt surface faces the flowing fluid, and partially obstructs the flow of the fluid. The partial obstruction of the flow induces vortices in the fluid. If the flow velocity exceeds a certain threshold, the induced vortices detach from the shedder and move downstream. Because the frequency of the vortices is proportional to the flow velocity of the fluid, a measurement of the frequency of the vortices can determine the flow velocity of the fluid. The tail portion is understood to stabilize formation of the vortices, and hence increase the accuracy of the flow velocity measurement.
Another aspect of the invention relates to manufacturing the vortex shedder as an integral unit by a precision casting process known in the art. The cast shedder includes an elongate portion having a blunt surface, a tail portion, and two mounting pads integrally joined to the elongate portion. The cast shedder typically requires machining of at least one of the mounting pads, to form a cavity or like recess for seating a sensor. One preferred embodiment provides at least one of the mounting pads of the vortex shedder with threaded holes for attaching other structures, such as a structure for dampening vibrations, to the shedder.
In one preferred embodiment of the invention, the two pads of the shedder are recessed to provide housings for two sensors. Such a shedder structure, herein referred to as a xe2x80x9cdouble-endedxe2x80x9d structure, allows mounting of two separate electronic modules for a xe2x80x9cdual-meterxe2x80x9d arrangement. Such a double-ended structure provides certain advantages including measuring flow rates of different types of fluids, such as liquid and steam, with a single flow meter. This and other advantages of a double-ended structure are described in more detail below.
In another embodiment, only one mounting pad of the shedder houses a sensor, and the other end of the shedder protrudes externally through one opening in the wall of the fluid-guiding conduit, and extends outwardly beyond the conduit wall. The protruding portion of the shedder can advantageously serve as a platform for stably resting the flowmeter on a flat surface, thus facilitating assembly, fixturing, and/or other handling of the flowmeter.
One preferred practice of the invention optionally provides structures, such as tapped holes, at least one end of the shedder for adding structures for dampening vibrations of the flowmeter and/or for providing additional structural support to the flowmeter. Such structures are particularly useful for dampening vibrations where piping systems to which the flowmeter is attached have excessive vibrations. The addition of such structures to the shedder of the present invention provides distinct advantages over conventional flowmeters that typically rely on external pipe supports to minimize vibrational effects.
In a preferred embodiment of the flowmeter of the invention, the two mounting pads at the opposite ends of the shedder have similar diameters to balance pressure forces acting on the shedder. In another embodiment, the two radially opposed openings in the wall of the passage guiding the process fluid are selected to have two different sizes. Such a difference in diameters of the openings allows insertion of a shedder, in which one mounting pad is smaller than the other and has a stepped or otherwise tapered periphery, into the passage. For example, one mounting pad can have a stepped diameter that includes two portions, both of which pass through one of the openings, but only one of which passes through the other opening. Thus, the stepped diameter provides a built-in stop that ensures proper positioning of the elongate portion of the shedder within the passage.
The mounting pads at the opposite ends of a shedder according to the present invention can have a variety of different cross-sectional geometries. For example, while some embodiments of the invention include mounting pads having circular cross-sections, other alternative embodiments include mounting pads with square cross-sections. Employing mounting pads having square or rectangular cross-sections can advantageously simplify the manufacturing of the shedder. For example, a surface of a square or rectangular bar can be selected to provide a contiguous surface including the blunt face of the shedder and a surface of each mounting pad, thereby facilitating production of the shedder.
The shedder of the invention can be installed in pipes have a variety of different inner diameters without machining of the shedder to size it for a particular pipe size. Thus, the present invention results in considerable savings of time and of labor in attaining flowmeters suited for a variety of applications.